Technical Field
Embodiments described herein relates to an exhaust gas processing system and a processing method for removing a sulfur oxide, a nitrogen oxide and the like from an exhaust gas containing carbon dioxide, such as combustion gas, to separate and recover carbon dioxide.
Description of the Related Art
In thermal power stations, ironworks, boilers and other facilities, fuels such as coal, heavy oil and extra heavy oil are used in a large quantity. Sulfur oxides, nitrogen oxides, and carbon dioxide discharged by the burning of the fuels need quantitative and concentration limitations with respect to the release thereof from the viewpoint of prevention of air pollution and conservation of global environment. In recent years, carbon dioxide has been regarded as a problem as it is the main cause of global warming, so that a movement of suppressing the emissions thereof in the world has been becoming activated. Thus, various researches have been actively promoted for making it possible to recover and store carbon dioxide from combustion exhaust gas or process exhaust gas without discharging carbon dioxide into the atmosphere. Combustion exhaust gas contains not only carbon dioxide and water but also nitrogen oxides, sulfur oxides, mercury, hydrogen chloride, ash dusts (particulate matters) and the like as minor components. It is therefore important from the viewpoint of environmental conservation to decrease the quantity of impurities contained in the carbon dioxide recovered from the exhaust gas to increase the purity of carbon dioxide.
Of the nitrogen oxides contained in combustion exhaust gas, nitrogen dioxide is removable by a wet absorption processing using an alkaline agent. However, nitrogen monoxide is poorly soluble in water. Thus, many of ordinarily performed denitration techniques are based on a dry-type ammonia catalytic reduction method, and nitrogen oxides are reduced by catalytic reaction by supplying ammonia or some other hydrogen source. When a desulfurization and denitration apparatus is formed on the basis of such a technique, in its desulfurization unit, sulfur oxides in an exhaust gas are processed in the state of ammonium salts.
In the meantime, about the desulfurization methods, various wet or dry processing techniques have been researched for removing sulfur oxides, using an alkaline desulfurizing agent. For example, Publication Document 1 listed below describes an exhaust gas wet processing method of bringing the exhaust gas and slurry containing a desulfurizing agent into liquid-gas contact with each other, in which carbon dioxide is recovered by desulfurization of the exhaust gas. Examples of the alkaline agent usable in such a desulfurization method include sodium hydroxide (or sodium carbonate), limestone (or slaked lime or dolomite), and magnesium hydroxide. Although sodium hydroxide is very high in efficiency of removing the sulfur oxides, it is expensive to increase costs for the processing. It is therefore general to apply, to large-sized plants such as thermal power stations, a limestone/gypsum method, in which limestone (calcium carbonate) or slaked lime (calcium hydroxide), which is inexpensive, is used.
As a method in which a hydrogen source or a desulfurizing agent as described above is not used, suggested is a method of pressurizing the exhaust gas, and then cooling it to condense the water content in the exhaust gas (see Publication document 2 listed below). In this method, sulfur oxides and nitrogen oxides contained in the pressurized exhaust gas are dissolved in the condensed water, and denitration and desulfurization of the exhaust gas are performed by separating the condensed water from the exhaust gas.